1. Field of the Invention
The present invention relates to a damping device for damping a vibration of a structure, such as an aerospace structure including a spacecraft and an aircraft, or a transportation structure including an automobile and an electric train.
2. Description of the Related Art
Generally, a vibration occurring in a structure significantly hinders an intended function of the structure, in most situations.
In observation or imaging of a far-off astronomical object with an extremely high spatial resolution in outer space, or in a spacecraft having a mission to take an image of the earth's surface with an extremely high spatial resolution, a vibration in an observational structure, a support structure thereof or a major structure of the spacecraft becomes a major obstacle to achieving the mission. A vibration in a precision instrument requiring accurate positioning becomes a major obstacle to offering its original performance. Further, a vibration in a building or in a transport machine, such as a vehicle, causes discomfort of users. Thus, there is the need for suppressing these vibrations.
A piezoelectric element serves as a means to bi-directionally convert between vibrational energy and electric energy, and has a property of generating a voltage in response to a force or a strain applied thereto (i.e. piezoelectric effect), and, inversely, generating a strain or a force in response to an electric charge or a voltage applied thereto (i.e. inverse piezoelectric effect). By taking advantage of these effects, the piezoelectric element is widely used as actuators and sensors for vibration control or damping. A damping system for suppressing a vibration is roughly classified into three systems: an active damping system, a passive damping system and a semi-active damping system.
As one example of an active damping system using a piezoelectric element, there has been known a system in which an actuator comprising a piezoelectric element is installed onto a structure, and a voltage or an electric charge is applied to the actuator in such a manner that the piezoelectric element is driven to generate a force for allowing the actuator to cancel a vibration. That is, with the active damping system, some energy may be temporarily supplied to the vibration system from the controller. This fact indicates the possibility of instability of the vibrating system with a poorly designed active damping system.
Typically, the passive damping system is intended to suppress a vibration based on an energy-dissipation effect of a structure itself or a device mounted on a structure. As a passive damping system using a piezoelectric element, there has been known a system designed to convert vibrational energy of a structure into electric energy through a piezoelectric element mounted on the structure, and dissipate the electric energy based on an electric resistance of a shunt circuit connected to the piezoelectric element. There has also been known a damping system using a resonance circuit including a coil to achieve enhanced damping efficiency, so-called “piezoelectric shunt damping system”.
The piezoelectric shunt damping system is presented, for example, in the following Non-Patent Publications 1 to 3.
FIG. 1 is a circuit diagram showing a fundamental principle of a damping device based on the piezoelectric shunt damping system. In FIG. 1, the reference numeral 2 indicates a piezoelectric element; 4 indicates a coil; and 8 indicates a resistor.
The passive damping system has no need for supply of external energy, and therefore can maintain operational stability. In contrast, its damping performance is generally not so high.
As one example of a semi-active damping system using a piezoelectric element, there has been known a system using a shunt circuit consisting of a piezoelectric element, a resistor and a switch, and a controller adapted to selectively open and close the switch depending on a vibration phase, as presented, for example, in the following Non-Patent Publication 4. This system is designed to effectively convert vibrational energy into electric energy and dissipate the electric energy based on the resistor, according to the switching operation of the switch.
Specifically, in the damping system presented in the Non-Patent Publication 4 by Richard et al., when a displacement or strain of a structure or a voltage generated by a piezoelectric element 2 has a positive or negative extreme value, due to a vibration of the structure, a switch 3 inserted in a piezoelectric element-based shunt circuit illustrated in FIG. 2 is closed, and kept in the closed state only in a short period where an electric charge accumulated in the piezoelectric element 2 is substantially fully discharged. The switch 3 is maintained in an open state during other period. The Non-Patent Publication 4 demonstrates that this system achieves enhanced damping performance as compared with a system using no switch.
As another example of the semi-active damping system using a piezoelectric element, there has been known a system using an electric circuit consisting of a piezoelectric element 2, a coil 4, a switch 3 and a resistor 8, as shown in FIG. 3, and a controller adapted to drive the switch 3 in such a manner that the switch 3 is closed when a displacement of a structure or a voltage generated by the piezoelectric element 2 has a positive or negative extreme value, due to a vibration of the structure, and opened just after a current flowing between the electrodes of the piezoelectric element 2 subsequently is reduced to zero, as presented, for example, in the following Non-Patent Publication 5 by Richard et al., and the following Patent Publication 1. Further, the following Patent Publication 2 presents a system using an improved circuit illustrated in FIG. 4 and proposes an improved switch control rule. In FIG. 4, the reference numeral 2 indicates a piezoelectric element; 4 indicates a coil; 8 indicates a resistor; 9 indicates a switch; 10 indicates a diode; and 11 indicates a diode. The Non-Patent Publication 5 and the following Non-Patent Publication 6 report on the result that the shunt circuit illustrated in FIG. 3 or 4 is operable to accumulate electric energy converted from vibrational energy in the piezoelectric element and the shunt circuit and allow the accumulated energy to be effectively utilized for vibration suppression so as to obtain a particularly-high damping performance.
Generally, these semi-active damping systems provide a higher damping performance as compared with the passive damping systems. In addition, the semi-active damping systems have no need for supply of external energy, and therefore can maintain operational stability, except for an electric power required for driving a control circuit adapted to measure a voltage generated by the piezoelectric element and selectively open and close the switch in the shunt circuit based on the measured information although an amount of the required power is generally small.
[Patent Publication 1] France Patent No. 2828256
[Patent Publication 2] Japanese Patent Laid-Open Publication No. 2004-132533
[Non-Patent Publication 1] Hagood, N. W and von Flotow, A., Damping of Structural Vibrations with Piezoelectric Materials and Passive Electrical Networks, J. Sound and Vibration, 146, 2, 243, 1991
[Non-Patent Publication 2] Hagood, N. W and Crawley, E. F., Experimental Investigation of Passive Enhancement of Damping for Space Structures, J. Guidance, Control and Dynamics, 14, 6, 1100, 1991
[Non-Patent Publication 3] Wu S., Piezoelectric shunts with a parallel R-L circuit for structural damping and vibration control, Proceedings of the International Society for Optimal Engineering, Vol. 2720, pp. 259-269, 1996
[Non-Patent Publication 4] Richard, C., Guyomar D., Audigier, D., Ching, G., Semipassive damping using continuous switching of a piezoelectric device, Proc. SPIE Conf. On Damping and Isolation, Newport Beach, Calif., Mach 1999, SPIE Vol. 3672, pp. 104-111
[Non-Patent Publication 5] Richard, C., Guyomar, D., Audigier D., Bassaler H., Enhanced semi passive damping using continuous switching of a piezoelectric device on an inductor, Proc. SPIE Conf. On Damping and Isolation, Newport Beach, Calif., Mach 2000, SPIE Vol. 3689, pp. 288-299
[Non-Patent Publication 6] Onoda, J., Makihara, K., Minesugi, K., Energy-recycling Semi-Active Method for Vibration Suppression with Piezoelectric Transducers, AIAA Journal, Vol. 41, No. 4, 2003, pp. 711-719